The objective of mechanical pulping is to produce high-yield pulps. Several years ago mechanical pulping was limited to a single process, the grinding of roundwood against a pulpstone, but since then mechanical pulping has expanded into an array of processes that use chemical, thermal and compression technologies (Casey, J. P. [1983] Tappi Journal 66:95-96). A drawback to the current methods used is that they produce pulp with poor bonding strength and poor brightness stability.
Thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP) and chemimechanical pulp (CMP) processes have evolved to improve mechanical pulp quality, expanding its utility in end product applications. Thermomechanical pulping is the dominant alternative high-yield pulping process. Its major limitation is the requirement for high electrical energy input, most of which ends up as low grade heat.
The utilization of thermomechanical pulps would be greatly facilitated if there was an increase of strength properties and if the stability of brightening could be enhanced, i.e., prevent brightness reversion. Brightness reversion of commercial pulps can be related to the presence of oxidized groups. These groups are principally derived from the residual breakdown products of lignin. It is postulated that the introduction of aldehyde and ketone groups into cellulose upon bleaching also contributes to brightness reversion, although to a lesser extent (Springer, E. L. [1983] Tappi Journal 66:93-96.). Breakdown products of lignin cause brightness reversion by mechanisms that are now being elucidated in several laboratories. It has been postulated that .alpha.-carboxyl groups adjacent to aromatic rings in residual lignin absorb daylight and transfer this energy to oxygen which in turn reacts with the phenolic groups of the lignin leading to formation of colored (yellow) quinones (Rapson, W. H. [1969] Appita 23:102-114). This reaction can occur only on "exposed" lignin rings which contain a free hydroxyl group.
Foremost in preventing brightness reversion is the necessity to modify lignin by oxidative cleavage of the exposed aromatic rings so that they cannot form quinones. This should drastically reduce brightness reversion properties.
Coarse TMP can be produced with relatively low inergy input. Subsequent secondary refining, however, requires substantial energy for development of pulp properties (Higuchi, T. [1982] Experientia 38:159-166). Experiments have demonstrated (Pilon, L., Desrochers, M., Jurasek, L., Neuman, P. J. [1982] Tappi Journal 65:93-96) that treatment of coarse TMP with P. chrysosporium cultures for 14 days can substantially reduce the energy requirement for secondary refining without a loss in pulp quality. Preliminary studies showed that the energy requirements to develop a given freeness in fungal-treated pulp was reduced by 25-30% as compared to untreated pulps Furthermore, pulp properties, as measured by the burst index, were also improved considerably. Because the refining of mechanical pulps after swelling in alkali can ccnsiderably improve strength properties, both the fungus-treated and untreated pulps were subjected to refining after swelling in alkali. The fungus-treated pulp then required 50% less refining energy than did the untreated pulp without any loss in strength properties.
In related experiments (Alberti, B. N. and Klibanov, A. M. [1981] Biotech. and Bioeng. Symp. 11:373-379), the water retention value (WRV) of fungal-treated pulps was tested in order to evaluate the effects of biological treatment on mechanical pulps. The WRV is a measure of the swelling and flexibility of the fibers. Increased swelling indicates greater contact between fibers during papermaking, this increasing the strength. An 88% gain in WRV was obtained after pretreatment of pulp with Schizophyllum commune, another white rot fungus, over untreated pulps.
The technical problems in applying organisms to industrial mechanical pulps, including TMP processing, are threefold: (a) in scaling-up with the required careful control of humidity, aeration and temperature; (b) in preventing contamination by unwanted organisms; and (c) in the impractical slowness of lignin degradation.